In the coating of substrates, solventborne binders are increasingly being replaced by aqueous, environment-friendly systems. Binders based on polyurethane-polyurea dispersions are playing an increasing role, in particular, on the basis of their excellent properties.
In many sectors so-called post-crosslinkable PU dispersions are being used: The crosslinking mechanism of these systems is based on blocked isocyanate groups in combination with isocyanate-reactive groups.
The blocking of polyisocyanates for the temporary protection of the isocyanate groups is a long-known working method and is described for example in Houben Weyl, Methoden der organischen Chemie XIV/2, pp. 61–70. Curable compositions comprising blocked polyisocyanates find use, for example, in polyurethane coating materials.
An overview of blocking agents suitable in principle is found, for example, in Wicks et al. in Progress in Organic Coatings 1975, 3, pp. 73–79, 1981, 9, pp. 3–28 and 1999, 36, pp. 148–172.
The preparation of polyurethane dispersions containing blocked isocyanate groups is described in the literature for example in DE-A 195 48 030.
A disadvantage when using post-crosslinkable polyurethane dispersions of the prior art is that after deblocking and crosslinking a certain fraction of the blocking agent remains as a monomer in the resultant coating film and adversely effects its quality. Qualities such as scratch resistance and acid stability of one-component coating films are incomparable with those of two-component (2K) polyurethane coatings, owing to the residual blocking agent (e.g. T. Engbert, E. König, E. Jürgens, Farbe&Lack, Curt R. Vincentz Verlag, Hannover 10/1995).
Systems of this kind are prepared, with chain extension, in the range from 90 to 120° C., so that such crosslinking in aqueous systems can take place only with destabilization of the dispersion particles. The prior art methods of extending and crosslinking NCO prepolymers takes place by reaction of isocyanate-functional prepolymers with isocyanate-reactive components. Within the aqueous medium there is an uncontrolled and poorly reproducible reaction of water with the isocyanate groups, this reaction being all the more pronounced the more reactive the isocyanate used. Chain extension with amines, accordingly, is difficult to reproduce and, even where the reaction mixture has the same composition, can lead to dispersions having different pH values.
In the preparation of dispersions by the acetone method it is also possible to use reactive aromatic diisocyanates, but with that method it is necessary to use relatively large amounts of acetone, which must later on be removed by distillation in a laborious second reaction step.
The object of the present invention was to provide new hydrophilic polyurethane prepolymers which can be processed readily to aqueous polyurethane dispersions and which do not have the disadvantages known from the prior art.